The present invention relates to heat exchangers, and more particularly, to an indirect heat exchanger comprised of a plurality of tube run circuits. Each circuit is comprised of a tube having a plurality of tube runs and a plurality of return bends. Each tube may have the same surface area from near its connection to an inlet header to near its connection to an outlet header. However, the geometry of the tube run is changed as the tube runs extend from the inlet to near the outlet header. In one case, the horizontal cross sectional dimension of the tube runs decrease as the tube runs extend to near the outlet header. Such decrease in horizontal cross sectional dimension may be progressive from the near the inlet header to near the outlet header or each coil tube run may have a uniform horizontal cross sectional dimension, with at least one horizontal cross section dimension of tube runs decreasing nearer to the outlet header.
In particular, an indirect heat exchanger is provided comprising a plurality of circuits, with an inlet header connected to an inlet end of each circuit and an outlet header connected to an outlet end of each circuit. Each circuit is comprised of a tube run that extends in a series of runs and return bends from the inlet end of each circuit to the outlet end of each circuit. In the embodiments, the tube runs may have return bends or may be one long straight tube with no return bends such as with a steam condenser coil. Each circuit tube run has a pre-selected horizontal cross sectional dimension near the inlet end of each coil circuit, and each circuit tube run has a decreasing horizontal cross sectional dimension as the circuit tube extends from near the inlet end of each circuit to near the outlet end of each coil circuit.
The embodiments presented start out with a larger tube geometry either in horizontal cross sectional dimension or cross sectional area in the first runs near the inlet header and then have a reduction or flattening (at least once) in the horizontal cross-sectional dimension of tube runs proceeding from the inlet to the outlet and usually in the direction of airflow. A key advantage towards progressive flattening in a condenser is that the internal cross sectional area needs to be the largest where the least dense vapor enters the tube run. This invites gas into the tube run by reducing the internal side pressure drop allowing more vapor to enter the tube runs. The reduction of horizontal tube run cross sectional dimension, or flattening of the tube in the direction of air flow accomplishes several advantages over prior art heat exchangers. First, the reduced projected area reduces the drag coefficient which imposes a lower resistance to air flow thereby allowing more air to flow. In addition to airflow gains, for condensers, as refrigerant is condensed there is less need for interior cross sectional area as one progresses from the beginning (vapor-low density) to the end (liquid-high density) so it is beneficial to reduce the internal cross sectional area as the fluid flows from the inlet to the outlet allowing higher internal fluid velocities and hence higher internal heat transfer coefficients. This is true for condensers and for fluid coolers, especially fluid coolers with lower internal fluid velocities. In one embodiment shown, the tube may start round and the geometric shape is progressively streamlined for each group of two tube runs. The decision of how many tube runs have a more streamlined shape and a reduction in the horizontal cross sectional dimension and how much of a reduction is required is a balance between the amount of airflow improvement desired, the amount of internal heat transfer coefficient desired, difficulty in degree of manufacturing and allowable internal tube side pressure drop.
Typical tube run diameters covering indirect heat exchangers range from ¼″ to 2.0″ however this is not a limitation of the invention. When tube runs start with a large internal cross sectional area and then are progressively flattened, the circumference of the tube and hence surface area remain essentially unchanged at any of the flattening ratios for a given tube diameter while the internal cross sectional area is progressively reduced and the projected area in the air flow external to the indirect heat exchanger is also reduced. The general shape of the flattened tube may be elliptical, ovaled with one or two axis of symmetry, a flat sided oval or any streamlined shape. A key metric in determining the performance and pressure drop benefits of each pass is the ratio of the long (vertical) side of the oval to the shortest (horizontal) side. A round tube would have a 1:1 ratio. The level of flattening is indicated by increasing ratios of the sides. This invention relates to ratios ranging from 1:1 up to 6:1 to offer optimum performance tradeoffs. The optimum maximum oval ratio for each indirect heat exchanger tube run is dependent on the working fluid inside the coil, the amount of airside performance gain desired, the desired increase in internal fluid velocity and increase of internal heat transfer coefficients, the operating conditions of the coil, the allowable internal tube side pressure drop as well as the manufacturability of the desired geometry of the coil. In an ideal situation, all these parameters will be balanced to satisfy the exact need of the customer to optimize system performance, thereby minimizing energy and water consumption.
The granularity of the flattening progression is an important aspect of this invention. At one extreme is a design where by the amount of flattening is progressively increased through the length of multiple passes or tube runs of each circuit. This could be accomplished through an automated roller system built into the tube manufacturing process. A similar design with less granularity would involve at least one step reduction such that one or more passes or tube runs of each circuit would have the same level of flattening. For example, one design might have the first tube run with no degree of flattening, as would be the case with a round tube, and the next three circuit tube runs would have one level of compression factor (degree of flattening) and the final four tube run passes would have another level (higher degree) of compression factor. The least granular design would have one or more passes or tube runs of round tube followed by one or more passes or tube runs of a single level of flattened tube. This could be accomplished with a set of rollers or by supplying a top coil with round tubes and the bottom coil with elliptical or flattened tubes. Yet another means to manufacture the different tube geometric shapes would be to stamp out the varying tube shapes and weld the plates together as found in U.S. Pat. No. 4,434,112. It is likely that heat exchangers will soon be designed and produced via 3D printer machines to the exact geometries to optimize heat transfer as proposed in this invention.
The tube run flattening could be accomplished in-line with the tube manufacturing process via the addition of automated rollers between the tube mill and bending process. Alternately, the flattening process could be accomplished as a separate step with a pressing operation after the bending has occurred. The embodiments presented are applicable to any common heat exchanger tube material with the most common being galvanized carbon steel, copper, aluminum, and stainless steel but the material is not a limitation of the invention.
Now that the tube circuits can be progressively flattened thereby reducing the horizontal cross sectional dimension, it is possible now to extremely densify the tube run circuits without choking external air flow. The proposed embodiments thusly allow for “extreme densifying” of indirect heat exchanger tube circuits. A method described in U.S. Pat. No. 6,820,685 can be employed to provide depression areas in the area of overlap of the U-bends to locally reduce the diameter at the return bend if desired. In addition, users skilled in the art will be able to manufacture return bends in tube runs at the desired flattening ratios and this is not a limitation of the invention.
Another way to manufacture a change in geometries shape is to employ the use of a top and bottom indirect heat exchanger. The top heat exchanger may be made of all round tubes while the bottom heat exchanger can be made with a more streamlined shape. This conserves the heat transfer surface area while increasing overall air flow and decreasing the internal cross sectional area. Another way to manufacture a change in geometric shape is to employ the use of a top and bottom indirect heat exchanger. The top heat exchanger may be made of all round tubes while the bottom heat exchanger can be made with a reduction in circuits compared to the top coil. This reduces the heat transfer surface area while increasing overall air flow and decreasing the internal cross sectional area. As long as the top and bottom coils have at least one change in geometric shape or number of circuits, the indirect heat exchange system would be in accordance with this embodiment.
It is an object of the invention to start out with large internal cross sectional area tube runs then progressively reduce the horizontal cross sectional dimension of tube runs as they progress from the inlet to the outlet to reduce the drag coefficient and allow more external airflow.
It is an object of the invention to start out with large internal cross sectional area tube runs then progressively reduce the horizontal cross sectional dimension of the tube runs as they progress from the inlet to the outlet to allow the lowest density fluid (vapor) to enter the tube run with very little pressure drop to maximize internal fluid flow rate.
It is an object of the invention to start out with large internal cross sectional area tube runs then progressively reduce the horizontal cross sectional dimension of tube runs as they progress from the inlet to the outlet to allow for extreme tube circuit densification without choking external airflow.
It is an object of the invention to start out with large internal cross sectional area tube runs then progressively reduce the horizontal cross sectional dimension of tube runs as they progress from the inlet to the outlet to increase the internal fluid velocity and increase internal heat transfer coefficients in the direction of internal fluid flow path.
It is an object of the invention to start out with large internal cross sectional area tube runs then progressively reduce the horizontal cross sectional dimension of tube runs as they progress from the inlet to the outlet on condensers to take advantage of the fact that as the vapor condenses, there is less cross sectional area needed resulting in higher internal heat transfer coefficients with more airflow hence more capacity.
It is an object of the invention to start out with large internal cross sectional area tube runs then progressively reduce the horizontal cross sectional dimension of tube runs as they progress from the inlet to the outlet by balancing the customer demand on capacity desired and allowable internal fluid pressure drop to customize the indirect heat exchanger design to meet and exceed customer expectations.
It is an object of the invention to change a circuits tube run geometric shape at least once along the circuit path to allow simultaneously balancing of the external airflow, internal heat transfer coefficients, cross sectional area and heat transfer surface area to optimize heat transfer.
It is an object of the invention to change a plate coil's geometric shape at least once along the circuit path to allow simultaneously balancing of the external airflow, internal heat transfer coefficients, cross sectional area and heat transfer surface area to optimize heat transfer.